The purpose of this project is to clarify the relationship between structure and antitumor activity in the actinomycin series. This includes structure determination of some novel actinomycins and a study of the secondary structure (conformation) of various actinomycins by nuclear magnetic resonance techniques. Also, comparative physicochemical studies of different actinomycins in complexation with nucleotides will be undertaken. The actinomycin variants under study include several compounds produced by directed biosynthesis, in which one or both proline residues have been replaced by such proline analogues as azetidine-2-carboxylic acid, pipecolic acid, 4-methylproline, etc. Explanations are sought for the considerable variations in biological activity which accompany these comparatively minor structural alterations.